This invention relates to a method and apparatus for measuring vibrations.
In one class of apparatus for measuring rotation motion of an object about an axis, a member that is movably mounted to the object is vibrated at an angle to the axis of rotation of the object. The combined effect of rotation of the object and vibration of the member is measured.
In a prior art measuring instrument of this type, the vibrating members are tines of a tuning fork velocimeter, mounted parallel to the axis of rotation about which the measurements are to be made. The tines vibrate periodically together toward and away from that axis.
In this prior art device, the tines of the fork are able to move angularly with respect to the base. As the tines move toward and away from the axis of rotation and change the distribution of mass about the axis of rotation, forces are created on the tines at an angle to the direction of vibration. These forces slow the angular motion of the tines of the fork about the axis of rotation or speed them with respect to the base to conserve angular momentum in response to the forces.
The motion between the tines and the base caused by the vibrations of the tines and the influence of conservation of momentum upon the tines during this rotation is measured. Since the speeding up and slowing down of the tines with respect to the base is related to the velocity of rotation about the axis of rotation as well as to the periodic vibration, a signal is obtained indicating the velocity.
This prior art type of measuring instrument for angular velocities has a disadvantage in that it is not able to measure changes over a wide enough bandwidth nor to measure changes with sufficient precision for many uses.
In another class of instrument for measuring angular motion, a beam is vibrated at its resonant frequency at an angle to its axis of rotation. When it rotates about its axis, the Coriolis forces created by the combined vibration and rotation are measured by a piezoelectric device. This force is indicative of the angular motion about its axis.
In the prior art devices of this class, the vibrating member is a beam mounted at both ends of its nodal point and driven at its fundamental frequency. It is aligned with the axis of rotation. The output is generated by piezoelectric crystals which are compressed by the Coriolis force to provide a DC output against certain electrodes.
This prior art velocimeter has the disadvantages of lacking sensitivity to relatively low angular changes. Some of the sensitivity is lost by excessive damping forces which reduce Coriolis and angular acceleration and instead reflect force back into the plane of vibration. Moreover, the beams are caused to accelerate in the direction of the Coriolis force, thus causing a force opposite to the Coriolis force, thus reducing the Coriolis acceleration. This reduced acceleration reduces the amount of sensed signal by treating Coriolis acceleration or Coriolis force as purely a kinetic event rather than a purely kinematic event.